Rotors in high speed machines are generally operated in an environment where both mechanical frictional losses (windage) and electrical current losses (eddy currents) contribute to the heating of the rotor during operation. Therefore cooling of the rotor in such machines is an important factor when considering their design.
In a radial flux machine, the rotor is often configured as a drum, rotating inside an annular stator assembly, with shaft bearings at either end of the drum. Rare earth permanent magnets are typically mounted to the shaft of the rotor and project an electromagnetic field radially outwards from the drum in a pattern determined by the number and shape of the magnetic poles. As these machines operate at high surface speeds (over 150 m/s), it is generally required to place a retaining sleeve around the magnets to keep them in place on the shaft. The selection of material for this sleeve is driven by a requirement for low density, high stiffness and high strength but also by a requirement for high longitudinal electrical resistivity, to minimise the electrical losses that manifest themselves in the sleeve due to its close proximity to the stator windings.
These electrical losses are primarily due to harmonic currents in the stator and are controlled by careful design of the electromagnetic circuit. However, any material that sits in the air gap between the magnets on the rotor and the stator will be subject to these harmonics, generating losses due to eddy currents in these materials. Accordingly, a carbon fibre composite is often selected as the most suitable material for the retaining sleeve. However, even with the use of a high resistivity carbon fibre sleeve, there is still a need to consider the cooling of the magnet material and any pole spacers that sit between the magnets, as these will also be subject to heat build up due to electrical losses.
It is known to provide cooling air to the rotor by forcing air to flow through the small radial gap between the outer diameter of the rotor sleeve and the bore of the stator. The size of this gap is important to both the electromagnetic performance of the machine as well as in determining the magnitude of the friction losses between rotor and stator. These considerations mean that the air gap is preferably as small as possible without unduly increasing friction to an unacceptable level.
Problems can therefore occur when the rotor is relatively long, as this leads to a pressure gradient building up over the length of the rotor. A higher pressure ratio is therefore required to move sufficient air through the gap. In addition, this will mean that the cooling air is continuously heated from one end to the other, and hence the components within the rotor will see a temperature gradient from one end to the other. Higher temperatures will adversely affect the performance of both the permanent magnets and the carbon fibre sleeve, and therefore the temperature at the hot end of the shaft will be the determining factor in the required flow rate of cooling air. Increasing the mass flow rate of cooling air through the gap to limit the temperature rise will normally cause the friction losses to increase, compounding the problem.
An additional problem with the known methods of cooling rotors in high speed machines is that the carbon fibre sleeve acts as a thermal insulator for the magnets and rotor structure underneath. Therefore, any heat produced from electrical losses in the rotor under the sleeve will be more difficult to remove using the air forced over the outer diameter of the sleeve. In addition, with known methods, the pressure ratio required to move sufficient cooling air through the radial air gap needs to be generated from an external source such as a cooling fan or by virtue of other means, which increases the cost and complexity of the system.